In conventional multi-mode filter, coupling elements is located in coupling windows on a wall to generate coupling between resonators.
The conventional multi-mode filter may obtain sufficient coupling amount in single mode, but can't realize sufficient coupling amount in multi-mode. Consequently, the conventional multi-mode filter has been used only as a narrow band filter.
In addition, if manufacturing of the conventional multi-mode filter is completed, coupling value of the conventional multi-mode filter can't tune any more.